The role of the mitochondria in thyroid hormone action has received support from our finding of a high affinity, low capacity binding component from mitochondria with the highest affinity for triiodothyronine (T3) of any subcellular moiety (Ka equal 2 times 10 to the 11th power L/M). (PNAS 72:3225, 1975). Further characteristics of this proposed hormone receptor have now been delineated. (1) Binding of analogues is releated to known physiological potency. (2) The receptor exhibits stereospecific discrimination (of L-T4 vs. D-T4). (3) Binding properties in vitro have been duplicated by intravenous administration of (I125)-T3 in vivo 30 minutes prior to sacrifice of the rat. (4) The saturable receptor was found in the mitochondrial membranes of rat liver, kidney, and myocardium, but not in mitochondria from the unresponsive tissues, brain, spleen and testis. (5) Fractionation of rat liver mitochondria into matrix, outer membrane, inner membrane, and inter-membrane protein has revealed the saturable macromolecule present only in the latter two moieties. (6) Analytic studies suggest a phospholipid containing lipoprotein of molecular weight approximating 150,000 daltons. (7) Oxidative phosphorylation of mitochondrial vesicles from hypothyroid rats has been stimulated from 110 percent to 150 percent of control by 2 minutes of incubation in buffered medium containing 5 mM succinate, 1mM ADP, and 1 mM phosphate on addition of T3 in the 1 pM-500 nM range, interpreted as a physiological stimulation of AT32P formation. High levels of T3 (100 micrometers) or dinitrophenol (DNP), a known uncoupler of oxidative phosphorylation, resulted in P32 incorporation much lower than in controls. The findings lend support to mitochondrial activation by T3 via a receptor associated with the inner membrane. Further studies are proposed to isolate and characterize the inner membrane receptor and its functions. BIBLIOGRAPHIC REFERENCE: Sterling, K. and P.O. Milch (1976). The mitochondria as a site of thyroid hormone action. In Thyroid Research. J. Robbins and L.E. Braverman, editors. Elsevier/North Holland, Amsterdam, 1976, pages 342-346.